First Input Delay Signals Test
The First Input Delay Test measures how long the browser takes to respond to the user's first interaction — a tap, click or keypress — after page load. The metric has been replaced in Core Web Vitals by Interaction to Next Paint (INP), which measures every interaction across the page lifetime. We report both: legacy FID for compatibility and INP as the modern signal Google now ranks against. Long delays almost always come from heavy JavaScript blocking the main thread.
What This Tool Checks
- Simulated FID measurement (first input)
- INP estimation across simulated interactions
- Long tasks (over 50 ms) on the main thread
- Third-party JavaScript impact on responsiveness
- Total Blocking Time (TBT) lab metric
- Hydration cost on JavaScript-rendered pages
Why It Matters for SEO
INP is the new Core Web Vital that replaced FID in March 2024. Google ranks pages by INP — anything over 200 ms is "Needs Improvement" and over 500 ms is "Poor". Slow interactivity comes from JavaScript hogging the main thread; common causes are heavy hydration on SSR frameworks, large React component trees, and third-party tag managers running on every interaction. Fixing INP usually means breaking up long tasks and removing or deferring third parties.
How to Fix It
Audit the long tasks in this report. Break them up using requestIdleCallback or yield to the main thread between expensive operations. Defer or remove third-party scripts that fire on every interaction. Use code splitting so only the JavaScript needed for the current page is loaded. Move expensive work off the main thread with web workers.
How It Works
We launch the page in headless Chrome, simulate user interactions at common touch points (CTA, form fields, navigation links) and measure how long each takes to produce the next paint. Long tasks during page load and during interaction are surfaced separately because each requires a different fix.
Common Mistakes to Avoid
- Heavy hydration on every page even when no interaction needed
- Third-party tag managers running synchronously on every click
- Long synchronous tasks blocking the main thread
- Event handlers that perform expensive work synchronously
- Treating INP as a one-off rather than measuring across the full session
Quick Checklist
- INP under 200 ms across all interactions
- No main-thread tasks longer than 50 ms
- Total Blocking Time under 200 ms
- Third-party scripts deferred
- Hydration cost minimized on SSR pages